Not applicable.
Not applicable.
The present invention relates to motor controllers and more particularly, to a method and an apparatus for altering stator winding voltages to eliminate greater than twice over voltage.
Many motor applications require that a motor be driven at various speeds. Motor speed can be adjusted with an Adjustable Speed Drive (ASD) which is placed between a voltage source and an associated motor that can excite the motor at various frequencies. One commonly used type of ASD uses a three-phase Pulse Width Modulated (PWM) inverter and associated PWM controller which can control both voltage and frequency of signals that eventually reach motor stator windings.
A three-phase PWM controller receives three reference or modulating signals and a triangle carrier signal, compares each modulating signal to the carrier signal and generates firing signals consisting of a plurality of pulses corresponding to each modulating signal. When a modulating signal has a greater instantaneous amplitude than the carrier signal, a corresponding firing signal is high producing a pulse on-time. When a modulating signal has an instantaneous amplitude that is less than the carrier signal, a corresponding firing signal is low producing a pulse off-time.
The firing signals are used to control the PWM inverter. A three-phase PWM inverter consists of three pairs of switches, each switch pair including series arranged upper and lower switches configured between positive and negative DC power supplies. Each pair of switches is linked to a unique motor terminal by a unique supply line, each supply line is connected to a node between an associated pair of switches. Each firing signal controls an associated switch pair to alternately connect a stator winding between the positive and negative DC power supplies to produce a series of high frequency voltage pulses that resemble the firing signals. A changing average of the high frequency voltage pulses over a period defines a fundamental low frequency alternating line-to-line voltage between motor terminals that drives the motor.
Insulated Gate Bipolar Transistors (IGBTs) are the latest power semiconductor switches used in the PWM inverter, IGBTs have fast rise times and associated switching speeds (e.g. 50-400 ns) that are at least an order of magnitude faster than BJTs and other similar devices. At IGBT switching speeds, switching frequency and efficiency, and the quality of terminal voltages, are all appreciably improved. In addition, the faster switching speeds reduce harmonic heating of the motor winding as well as reduce audible motor lamination noise.
While IGBT PWMs are advantageous for all of the reasons identified above, when combined with certain switch modulating techniques (i.e. certain on/off switching sequences), IGBT fast dv/dt or rise times can reduce the useful life of motor components and/or drive to motor voltage supply lines. In particular, while most motors and supply lines are designed to withstand operation at rated line voltages for long periods and to withstand predictable overvoltage levels for short periods, in many cases, fast switch rise times causes overvoltages that exceed design levels.
For a long time the industry has recognized and configured control systems to deal with twice overvoltage (i.e. twice the PWM inverter DC power supply level) problems. As well known in the controls art, twice overvoltage levels are caused by various combinations of line voltage rise time and magnitude, imperfect matches between line-to-line supply cable and motor surge impedances, and cable length. Line voltage frequency and switch modulating techniques have little effect on twice overvoltage levels.
There is another potentially more damaging overvoltage problem that has not been satisfactorily dealt with. The second overvoltage problem is referred to herein as greater than twice overvoltage. Unlike twice overvoltage, greater than twice overvoltage is caused by faster IGBT switching frequencies and faster IGBT dv/dt rise times interacting with two different common switch modulating techniques, that result in overvoltage problems referred to as xe2x80x9cdouble pulsingxe2x80x9d and xe2x80x9cpolarity reversalxe2x80x9d.
Each of the double pulsing and polarity reversal problems are described in detail in U.S. Pat. No. 5,912,813 (hereinafter xe2x80x9cthe ""813 patentxe2x80x9d) which issued on Jun. 15, 1999, is entitled xe2x80x9cMethod and Apparatus for Controlling Reflected Voltage Using A Motor Controllerxe2x80x9d. The ""813 patent is incorporated herein by reference for its teachings regarding double pulsing and polarity reversal.
One way to mitigate the adverse effects of rise time induced motor overvoltages has been to design and construct relatively complex passive filter networks. Unfortunately addition of passive filter networks increases overall system design costs and implementation, requires excessive relatively expensive panel space within a system housing or cabinet and can lead to heating and other operating problems. In addition, unfortunately, passive filters limit carrier frequency selection.
One other solution to mitigate the adverse effects of rise time induced motor overvoltages has been to modify modulation and commutation software. Some of the more sophisticated techniques of this type include providing a motor controller that modifies firing pulses that are provided to an inverter in a manner calculated to eliminate greater than twice overvoltage switching sequences. When the period between two voltage changes is less than the period required for a substantially steady state voltage near zero to be reached, the period between the two voltage changes is increased. Where switching sequence results in greater than twice overvoltage due to polarity reversal, the switching sequence is altered to eliminate the possibility of greater than twice overvoltage.
Software correction solutions generally contemplates two different methods of altering the switching sequence referred to as the Maximum-Minimum Pulse Technique (MMPT) and the Pulse Elimination Technique (PET) methods. According to the MMPT method, when a PWM pulse has characteristics which could generate greater than twice overvoltage, the pulse width is altered so that its duration is set equal to or between the minimum and maximum pulse times allowed (i.e., the carrier period less a dwell time where the dwell time is the minimum period required to avoid overvoltage). Importantly, only pulses that cross the threshold level for double pulsing induced motor voltages greater than twice overvoltage and during polarity reversal periods are altered so that the resulting terminal voltage magnitude is only minimally affected.
According to the PET method, instead of only limiting pulses to within the maximum and minimum pulse times, some of the pulses having characteristics which could generate greater than twice overvoltage are eliminated. In other words, some of the positive pulse durations during positive half cycles are increased and set equal to the carrier period and some of the negative pulse durations during negative half cycles are increased and set equal to the carrier period. The result is a terminal voltage magnitude which is essentially unaffected by pulse alterations.
Unfortunately each of the MMPT and PET methods alone do alter the resulting terminal voltages. For example, when an MMPT method is employed the terminal voltage magnitude is noticeably reduced as some positive pulse durations during positive half cycles and some negative pulse durations during negative half cycles are reduced. Similarly, when a PET method is employed the terminal voltage magnitude is noticeably increased as some positive pulse durations during positive half cycles and some negative pulse durations during negative half cycles are increased.
One way to deal with errors caused by MMPT and PET methods is to provide feedback loops in the control system. For example, one control system including a feedback loop has been operated with an 18 microsecond dwell time and a carrier frequency of between 1 and 12 kHz with insignificant distortion.
In the case of relatively less expensive open loop control systems the industry has developed additional correction software designed to counter the effects of MMPT and PET methods. While this software works well, optimal correction algorithms typically require a large number of calculations and hence relatively fast processors (e.g., (microprocessor, micro-controller, or hardware programmable device)
Where cost constraints limit processor capabilities, often correction software leads to disturbances at the point at which pulse widths are altered (i.e., the xe2x80x9cinception pointxe2x80x9d) with an attendant increase in current distortion. For example, referring to FIG. 1, a U-phase to positive bus voltage signal and resulting U-phase current waveform are illustrated that were generated using a controller employing a two phase discontinuous modulating waveform at 55 Hz with a 630 V bus, a carrier frequency of 4 kHz and a dwell time of ten microseconds. FIG. 1 clearly shows that the current waveform is distorted despite the fact that the corrective code for dealing with the reflected wave phenomenon has been activated.
FIG. 2 illustrates the frequency spectra for voltage U-phase to positive bus voltage and the U-phase current. Flattening of the current peak in FIG. 1 is consistent with the current spectrum of FIG. 2 where the 5th harmonic is approximately 0.5 Arms and he 7th harmonic is approximately 0.25 Arms. The voltage spectrum in FIG. 2 includes voltage components at the 5th and 7th harmonics and a common mode componentxe2x80x94the 2nd harmonic component illustrated.
FIG. 3 includes a current waveform lu similar to the current waveform of FIG. 1, except that the carrier frequency and dwell time used to generate the waveform of FIG. 3 where 8 kHz and 8 microseconds, respectively (the decrease in dwell time was necessary because of stability and distortion requirements). Clearly with the higher carrier frequency the current distortion is appreciably increased. The primary cause of the increased distortion is that the dwell time percent voltage increases as carrier frequency is increased. FIG. 4 is similar to FIG. 2 except that the spectrum of FIG. 4 correspond to a U-phase to positive bus voltage signal (not illustrated) generated with a carrier frequency and a dwell time of 8 kHz and 8 micro seconds, respectively, and the U-phase current waveform of FIG. 3. Clearly the magnitudes of the distorting harmonics are increased appreciably.
Given current constraints, controller designers are confronted with selecting among different control options and accepting various tradeoffs. Among the tradeoffs are current distortion vs. execution time and voltage feedback/speed range vs. cost.
Thus, it would be advantageous to have a method and apparatus that could be used to accurately eliminate greater that twice overvoltage that can be implemented via many different types of controllers including controller that are relatively minimally computationally capable.
It has been recognized that an accumulated error resulting from an imposed dwell time constraint from a previous PWM cycle can be added to a next pulse time to alter the next pulse time and hence substantially compensate for MMPT and PET induced distortion. This accumulation followed by error distribution is the key to a relatively simple software correction methodology that requires minimal execution time. While phase and magnitude error increases, the advantages associated with reduced execution time are appreciable and render the methodology most suitable for many control applications.
The invention includes a method to be used with a motor controller generating firing pulses to control an inverter, the inverter providing exciting voltage to a motor corresponding to the firing pulses, the voltage having a maximum intended amplitude, the method for substantially eliminating exciting voltage overvoltage by modifying the firing pulses, the method comprising the steps of (a)identifying characteristics of an initial firing pulse, (b)comparing the initial pulse characteristics to an overvoltage characteristic set known to cause overvoltage, (c)where the initial pulse characteristics match the overvoltage characteristic set, altering the initial firing pulse such that the altered firing pulse does not cause overvoltage, (d)identifying an accumulated error corresponding to the modified firing pulse, (e)modifying the firing pulse following the altered firing pulse as a function of the accumulated error to generate a composite firing pulse; and (f) repeating steps (a) through (e) with the composite firing pulse as the initial firing pulse.
In at least one embodiment the step of identifying the accumulated error includes identifying the difference between the initial firing pulse and the altered firing pulse. In some embodiments the step of modifying the firing pulse includes adding the accumulated error to the pulse following the altered firing pulse.
In some embodiments the controller includes a comparator that compares a reference signal to a carrier signal to provide the following firing pulses, one following firing pulse provided during each carrier period, each following firing pulse characterized by an on-time having a duration that is between zero and the duration of the carrier period, the step of comparing including comparing the duration to the initial firing pulse to zero, when the initial firing pulse duration is less than zero, the step of identifying an accumulated error including setting the accumulated error equal to the duration of the initial firing period and the step of altering the initial firing pulse including setting the duration of the altered firing pulse to zero.
In several embodiments the step of comparing further includes comparing the duration of the initial firing pulse to the carrier period duration, when the initial firing pulse duration is greater than the carrier period duration, the step of identifying an accumulated error including mathematically combining the initial firing pulse duration and the carrier period duration and the step of altering the initial firing pulse including setting the duration of the altered firing pulse to the carrier period duration.
The step of mathematically combining the initial firing pulse duration and the carrier period duration may include subtracting the carrier period duration from the initial firing pulse duration.
In several embodiments the overvoltage characteristic set includes a minimum pulse time corresponding to the minimum pulse durations that can occur without causing overvoltage to occur and, wherein, the step of comparing further includes, when the initial firing pulse duration is between zero and the carrier period duration, comparing the initial firing pulse duration to the minimum pulse time and, when the initial firing pulse duration is less than the minimum pulse time, the step of identifying an accumulated error including mathematically combining the initial firing pulse duration and the minimum pulse time and the step of altering the initial firing pulse including setting the duration of the altered firing pulse to the minimum pulse time. Here, the step of mathematically combining the initial firing pulse duration and the minimum pulse time may include subtracting the minimum pulse time from the initial firing pulse duration.
The overvoltage characteristic set may include a maximum pulse time corresponding to the maximum pulse durations that can occur without causing overvoltage to occur and, the step of comparing may further include, when the initial firing pulse duration is between zero and the carrier period duration, comparing the initial firing pulse duration to the maximum pulse time and, when the initial firing pulse duration is greater than the maximum pulse time, the step of identifying an accumulated error including mathematically combining the initial firing pulse duration and the maximum pulse time and the step of altering the initial firing pulse including setting the duration of the altered firing pulse to the maximum pulse time.
In one aspect the step of mathematically combining the initial firing pulse duration and the maximum pulse time includes subtracting the maximum pulse time from the initial firing pulse duration.
In some embodiments the controller includes a comparator that compares a reference signal to a carrier signal to provide the following firing pulses, each following firing pulse characterized by an on-time having a duration that is between zero and the duration of the carrier period, the overvoltage characteristic set including a minimum pulse time corresponding to the minimum pulse durations that can occur without causing overvoltage to occur and, wherein, the step of comparing includes the step of comparing the initial firing pulse duration to the minimum pulse time and, when the initial firing pulse duration is less than the minimum pulse time, the step of identifying an accumulated error including mathematically combining the initial firing pulse duration and the minimum pulse time and the step of altering the initial firing pulse including setting the duration of the altered firing pulse to the minimum pulse time.
In some embodiments the overvoltage characteristic set includes a maximum pulse time corresponding to the maximum pulse durations that can occur without causing overvoltage to occur and, wherein, the step of comparing further includes comparing the initial firing pulse duration to the maximum pulse time and, when the initial firing pulse duration is greater than the maximum pulse time, the step of identifying an accumulated error including mathematically combining the initial firing pulse duration and the maximum pulse time and the step of altering the initial firing pulse including setting the duration of the altered firing pulse to the maximum pulse time.
In several embodiments the controller includes a comparator that compares a reference signal to a carrier signal to provide the following firing pulses, each following firing pulse characterized by an on-time having a duration that is between zero and the duration of the carrier period, the overvoltage characteristic set including minimum and maximum pulse times corresponding to the minimum pulse durations and the maximum pulse durations that can occur without causing overvoltage and, wherein, the step of comparing includes comparing the initial firing pulse duration to the minimum pulse time and the maximum pulse time, when the initial firing pulse duration is less than the minimum pulse time, the step of identifying an accumulated error including setting the accumulated error equal to the initial firing pulse duration minus the minimum pulse time and the step of altering the initial firing pulse including setting the duration of the altered firing pulse to zero and, when the initial firing pulse duration is greater than the maximum pulse time, the step of identifying an accumulated error including setting the accumulated error equal to the initial firing pulse duration minus the maximum pulse time and the step of altering the initial firing pulse including setting the duration of the altered firing pulse to the carrier cycle duration.
The invention also includes a method to be used with a motor controller generating firing pulses to control an inverter, the inverter providing exciting voltage to a motor corresponding to the firing pulses, the voltage having a maximum intended amplitude, the controller including a comparator that compares a reference signal to a carrier signal to provide following firing pulses, one following firing pulse provided during each carrier period, each following firing pulse characterized by an on-time having a duration that is between zero and the duration of the carrier period, an overvoltage characteristic set including a minimum pulse time corresponding to the minimum pulse durations that can occur without causing overvoltage, the method for substantially eliminating exciting voltage overvoltage by modifying the firing pulses, the method comprising the steps of (a)identifying the duration of an initial firing pulse; (b)comparing the initial firing pulse duration to zero, (c)when the initial firing pulse duration is less than zero, setting an accumulated error equal to the duration of the initial firing period and setting the duration of an altered firing pulse to zero and skipping to step (k), (d)comparing the initial firing pulse duration to the carrier period duration, (e)when the initial firing pulse duration is greater than the carrier period duration, identifying an accumulated error by mathematically combining the initial firing pulse duration and the carrier period duration and setting the duration of an altered firing pulse to the carrier period duration and skipping to step (k), (f)comparing the initial firing pulse duration to the minimum pulse time, (g)when the initial firing pulse duration is less than the minimum pulse time, identifying an accumulated error by mathematically combining the initial firing pulse duration and the minimum pulse time and setting the duration of the altered firing pulse to the minimum pulse time and skipping to step (k), (h)comparing the initial firing pulse duration to the maximum pulse time, (i)when the initial firing pulse duration is greater than the maximum pulse time, identifying an accumulated error by mathematically combining the initial firing pulse duration and the maximum pulse time and setting the duration of the altered firing pulse to the maximum pulse time, (j)modifying the firing pulse following the altered firing pulse as a function of the accumulated error to generate a composite firing pulse and (k)repeating steps (a) through (j) with the composite firing pulse as the initial firing pulse.
Here, the step of mathematically combining the initial firing pulse duration and the carrier period duration may include subtracting the carrier period duration from the initial firing period duration, the step of mathematically combining the initial firing pulse duration and the minimum pulse time may include subtracting the minimum pulse time from the initial firing period duration and the step of mathematically combining the initial firing pulse duration and the maximum pulse time may include subtracting the maximum pulse time from the initial firing period duration.
Moreover, the invention includes an apparatus to be used with a motor controller generating firing pulses to control an inverter, the inverter providing exciting voltage to a motor corresponding to the firing pulses, the voltage having a maximum intended amplitude, the apparatus for substantially eliminating exciting voltage overvoltage by modifying the firing pulses, the apparatus comprising (a)a processor for identifying characteristics of an initial firing pulse, (b)a comparator for comparing the initial pulse characteristics to an overvoltage characteristic set known to cause overvoltage, (c)a first pulse modifier for, where the initial pulse characteristics match the overvoltage characteristic set, altering the initial firing pulse such that the altered firing pulse does not cause overvoltage and providing the altered firing pulse for inverter control, (d)an error identifier for identifying an accumulated error corresponding to the altered firing pulse and (e)a second pulse modifier for modifying the firing pulse following the altered firing pulse as a function of the accumulated error to generate a composite firing pulse, the composite firing pulse provided to the processor as the next initial firing pulse.
In some embodiments the error identifier identifies the accumulated error by subtracting the altered firing pulse duration from the initial firing pulse duration. In several embodiments the second pulse modifier modifies the firing pulse by adding the accumulated error to the pulse following the altered firing pulse.
In several embodiments the controller includes a comparator that compares a reference signal to a carrier signal to provide the following firing pulses, each following firing pulse characterized by an on-time having a duration that is between zero and the duration of the carrier period, the overvoltage characteristic set including a minimum pulse time and a maximum pulse time corresponding to the minimum and maximum pulse durations that can occur without causing overvoltage to occur and, wherein, the comparator compares the initial firing pulse duration to at least a subset of a zero value, the carrier period duration, the minimum pulse time and the maximum pulse time and when the initial pulse duration is greater than the maximum pulse time or less than the minimum pulse time, the first pulse modifier alters the initial pulse duration by subtracting one of the zero value, the carrier period duration, the minimum pulse time and the maximum pulse time from the initial pulse duration and wherein the error identifier identifies the accumulated error by, when the first pulse modifier alters the initial pulse duration by subtracting the zero value, the carrier period duration, the minimum pulse time or the maximum pulse time from the initial pulse duration, setting the accumulated error to a zero value, the carrier period duration, the minimum pulse time and the maximum pulse time, respectively.
In some embodiments when the initial pulse duration is less than zero, the first pulse modifier sets the altered pulse equal to the zero value, when the initial pulse duration is greater than the carrier period duration, the first pulse modifier sets the altered pulse equal to the carrier period duration, when the initial pulse duration is less than the minimum pulse time and greater than zero, the first pulse modifier sets the altered pulse equal to the minimum pulse time and when the initial pulse duration is greater than the maximum pulse time and less than the carrier period duration, the first pulse modifier sets the altered pulse equal to the maximum pulse time.
Furthermore, the invention includes an apparatus to be used with a motor controller generating firing pulses to control an inverter, the inverter providing exciting voltage to a motor corresponding to the firing pulses, the voltage having a maximum intended amplitude, the apparatus for substantially eliminating exciting voltage overvoltage by modifying the firing pulses, the apparatus comprising a processor running a program to perform the steps of (a)identifying characteristics of an initial firing pulse, (b)comparing the initial pulse characteristics to an overvoltage characteristic set known to cause overvoltage, (c)where the initial pulse characteristics match the overvoltage characteristic set, altering the initial firing pulse such that the altered firing pulse does not cause overvoltage, (d)identifying an accumulated error corresponding to the altered firing pulse, (e)modifying the firing pulse following the altered firing pulse as a function of the accumulated error to generate a composite firing pulse and (f)repeating steps (a) through (e) with the composite firing pulse as the initial firing pulse.
These and other objects, advantages and aspects of the invention will become apparent from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made therefore, to the claims herein for interpreting the scope of the invention.